1. Field of the Invention
The present invention generally relates to an image-capturing device of a scanner, and more particularly to a method for correcting the image information to reduce the image noise during scanning process.
2. Description of the Prior Art
In a digital reproducing apparatus or facsimile apparatus, an image reading apparatus having a photoelectric transducer such as CCD (charge-coupled device) is used to photoelectrically read an image of a document sheet. In such an image reading apparatus, a processing function called shading correction is provided in order to eliminate a scatter in a read output level due to a variance of sensitivity of an intensity of exposure light to the document sheet and a variance of sensitivity of the photoelectric transducer and uniformly read an image density.
In a prior art shading correction, a standard white plate of a know density is provided outside a document sheet read area, and the standard white plate is read prior to the reading of the document sheet and the read signal is stored in a memory. The scatter of the read output level can be detected by the signal stored in the memory. Accordingly, in order to eliminate the detected scatter, the shading correction is made to read data based on the data stored in the memory, when the document sheet is read.
In general, the read apparatus electrically main-scans the document sheet by using a linear CCD to read it and sub-scans it by mechanically moving the read line of the CCD transversely to the direction of the main scan. The standard white plate is located at a start of read position in the direction of sub-scan.
In the shading correction, a multiplication factor to render the light intensity data from the standard white plate for each pixel stored in the memory to a light intensity corresponding to the know density of the standard white plate is determined for each pixel and it is stored in the memory.
When the document sheet is read, the multiplication factor corresponding to the read pixel is read from the memory and it is multiplied to the document sheet read signal to effect the shading correction.
In the prior art, since the document sheet is mounted on a platen glass, the document sheet read signal is one transmitted through the platen glass. On the other hand, the standard white plate is formed by applying it on a thin metal plate and it is not always arranged at the same level as that of the document sheet but it is in many cases at a level of the bottom surface of the platen glass. Accordingly, the standard white plate is located closer to an illumination lamp than the document sheet is.
In addition, while the document sheet covered by the platen glass, the applied read plane of the standard white plate is exposed to the light source and the sensor device. As a result, in a long time use, the standard white plate exposed to the illumination significantly fades and the density level greatly deviates from the initially set level.
As a result, in spite of the fact that the shading correction is made on the assumption that the standard white plate has a know density, the signal derived by reading the standard white plate is not proper one due to a difference in the optical path lengths from the surface of the document sheet and a change in the density of the standard white plate.
On the other hand, it is an important drawback for the prior art is that the lamp brightness of the linear image scanner will be different due to the using time, the ambient temperature, and the variation position to obtain the image with noise. Further, the response is different for each sensor of linear sensor device so as to utilize to scan a standard white plate to obtain a standard curve. Therefore, the prior art is provided a method for scanning standard white plate is required to utilize prior to scan the document sheet to obtain the standard curve. Due to the scanning speed is increased to cause the duration of line exposing is shorted and the size of sensor device is reduced from 7 um to 4 um thereby the image noise will be increased, the output signal quality (S/N ratio) is descend, and the gain will be increased simultaneously. In addition, due to the lamp type (shaped or characteristic), the output of the two ends of lamp is smaller than the center of the lamp, and the output image noise will also be increased.
Referring to FIG. 1 that represents a flow diagram for the steps of the conventional image correction. According to flow diagram, step 1 is scanning the standard white plate to obtain a first image data; step 2 is calculating the first image data to obtain the shading curve (standard curve); step 3, according to step 2, we can obtain a shading curve corresponding to each sensor device; step 4, inputting the shading curve into the calibration memory with a calibration database; step 5, performing exposing and scanning process to the document sheet to obtain a second image data; step 6, reading out the shading curve corresponding to each sensor device; step 7, calculating the shading curve; and the step 8, outputted the corrected image data.
According to above mentioned, the calibration procedure only performing a calculation process to the shading curve from scanning the standard white plate, thus, the output signal still have error between the originally image and output image. Therefore, the noise and gain still a problem during the scanning the document sheet.